Projects

Plummer Creek Wetlands Restoration

Plummer Creek flows through this site but its channel has been straightened and the riparian area has been degraded to the point that it provided limited flood flow attenuation, habitat or water quality improvement (see Photograph 1). These values were to be restored primarily through the construction of two low check dams that would make use of the existing topography to provide for enhanced flood storage and sediment trapping as well as seasonal open water habitat. The check dams would allow for fish passage in order to meet the Tribe's goals for the restoration of native salmonid use of this stream. The "island flow deflector" was designed to block the existing stream channel and direct water entering the wetland site into the enlarged pond with flood flows being directed into a new meander channel that would lead to the upper check dam.

Photograph 2. Plummer Creek Wetland site, pond construction, October 15, 2002.

The Plummer Creek watershed above the wetland site is a relatively undisturbed area covering approximately 2,500 acres. The length of the watershed is just less than three miles, 15,300 feet, along the north-south axis. The lower elevation (at the project site) is approximately 2,715 feet, while the upper elevation at the south limit of the basin is 3,371 feet. The upper elevation at Plummer Butte is 4,145 feet. Thus the average slope within the watershed is between 4% and 9%.

General vegetation types in the watershed include approximately 600 acres of pasture or cultivated for hay production and approximately 1,900 acres of woods/forest or shrubs. There are fewer than ten homes or farmsteads in the watershed and few paved roads (US 95 being the primary exception). Considering the soils, slope and vegetation in this basin, the expected runoff from the design storm (25 year, 24 hour peak storm) is 0.43 inches in depth which results in a total runoff volume of 89.6 acre feet and a peak discharge of 159 cubic feet per second.

Figure 2. Plummer Creek / Wetland site layout (not to scale)

Following research into possible wetland restoration techniques, the "check dam" design was developed. This is an earthen embankment dam topped with a rocked two-stage broad crested weir. This is a self-regulating structure designed to temporarily retain flood flows thereby raising the soil water content. Based on the topography of the site, two check dams were desired. Due to the lack of soil available on or near the wetland site that could be used to build the check dams, soil was to be imported; further, that soil had to contain a clay content (approximately 20% by weight) that was appropriate for compaction in the embankment. Fortunately, the planned transport of rock from a commercial pit near Plummer to a streambank restoration site in the Hangman Creek watershed provided an opportunity to utilize trucks to bring such clay soils back on their return trip.

While there was an existing topographic survey map that had been prepared showing the entire Industrial Park site, this did not show sufficient detail of the wetland features to be used for the final design of the check dams. As a result, a level survey was conducted which focused on the elevations of the stream channel bottom and the ground surfaces where the two check dams were to be placed. This elevation information was tied into the existing survey datum and allowed cross section and longitudinal profiles to be plotted on which the check dams could be shown.

Photograph 3. Plummer Creek Wetland site, island flow deflector, October 24, 2002.

The next important task to be performed on this project was a vegetation survey of the project site and an upstream reference site. These were necessary to establish the existing condition in the area to be restored and also provide targets for species distribution in the completed project. The vegetation survey was performed as a walkthrough of the entire site with identification of all visible plant species. At the same time plants were surveyed, an estimation of their coverage was made as a means of establishing the dominant species. Because there were differences apparent in the predominant vegetation types found on different parts of the wetland site, the survey was divided into five areas and the coverages recorded referred to the given “Veg. Area” only, not the entire site. Finally, a wetland indicator status was determined for each identified plant species. These indicators provide some insight into the extent that each area is currently supporting key wetland species.

In order to describe the Plummer wetland restoration process for potential reviewers or interested parties, the Plummer Industrial Park Wetland Restoration Plan was prepared. The basic content of this Plan followed the 1993 Washington Department of Ecology publication entitled: Restoring Wetlands in Washington; A Guidebook for Wetland Restoration, Planning and Implementation. This included a detailed site assessment for both the project site and the upstream reference site. Features of the site assessment included topography, soils, ecological communities (vegetation, exotic species, wildlife and fish), hydrology, boundary delineation, water quality and existing wetland values and functions. Following the site assessment, design criteria were explained. Design criteria included performance standards and specific design features of the various desired structures. The Restoration Plan also addressed general procedures for maintenance of constructed features and basic monitoring efforts to allow documentation of changes in the wetland system over time and success of the restoration techniques.

Photograph 4 Plummer Creek Wetland site, constructing check dam across existing creek channel October 21, 2002.

Following completion of the Restoration Plan, a Joint Application for Permits was prepared and submitted to the US Army Corps of Engineers. Also, in support of hiring a contractor to construct the desired restoration features, Plans and Specifications and bid solicitation documents were prepared. Included with these documents, a site map (see Figure 2) showed the location of the proposed restoration features.

Work performed on this project in 2002 included the preparation of a bid solicitation for the construction work; touring potential contractors through the project site; improvement of the access into the site using Tribal and Plummer Forest Products (operators of the adjacent log yard) staff and equipment; selection of a contractor (Professional Operators Company of Lewiston, ID) and pre-construction surveying and staking. Actual Construction work began October 15 and was completed in approximately two weeks. Heavy equipment used included a 315B CAT Excavator (with 40" toothless bucket and thumb), 550 G John Deer Dozer and 10 cubic yard dump truck.

The construction work progressed generally from the upstream end of the project downstream, starting with the deepening of the existing pond (see Photograph 2). The predominantly clay soil removed from this location was trucked and spread on the upland area to the west of the wetland boundary. The new depth of the pond is 10 feet below the surrounding ground surface and this depth meets a layer of gravelly clay. There was a small amount of water entering the excavation from approximately the six-foot depth. The surface dimensions of the finished pond are approximately 80 feet north to south and 60 feet east to west.

Photograph 5. Plummer Creek Wetland site, constructing spillway in upper check dam, October 24, 2002.

Next, two new channels were dug to connect the existing channel with the pond; one for inflow one for outflow. Then the "island" could be constructed, this being simply an earthen plug in the channel. It was necessary to pump the water out of the channel above the island to allow the placement of geotextile and the rock face of the island which constituted the "flow deflector" (see Photograph 3).

The construction of the upper check dam involved digging a two foot by four foot core trench the length of the dam, including across the existing stream channel, and then filling / compacting this trench with imported soil. This was done progressively working east to west. The core trench across the stream channel involved placing a temporary plug in the channel both above and below the core trench location and pumping the water out between these (see Photograph 4). Since the imported soil was too dry for acceptable compaction, this creek water was pumped (sprayed) into the trench as the soil was being pushed in with the dozer, so it did not need to be disposed elsewhere. Compaction of core trench and embankment dam was accomplished using the dozer.

Photograph 5. Plummer Creek Wetland site, constructing spillway in upper check dam, October 24, 2002.

The construction of the embankment followed the completion of the core trench and this was completed on October 22nd. Then the compacted material was removed at the spillway location. The toothless bucket was invaluable for this because it allowed a precise cut in the shape and dimensions of the spillway. On this cut a non-woven geotextile was placed and then a layer of rock, following the gradation provided in the project specifications (see Photograph 5).

The lower check dam was constructed by shaping the existing fill that formed a vehicle access from the adjacent log yard. This fill was a mixture of soil and small rock and was extremely tightly compacted. However, the existing 36-inch culvert through this fill had to be removed and this excavation was re-filled and compacted with a mixture of the soil / rock material and imported clay soil. Compaction in this area was achieved using the excavator bucket. The spillway through the lower dam was again cut out of the fill and covered with geotextile and rock.

Photograph 7. Plummer Creek Wetland site, footbridge construction and plantings near upper check dam, December 20, 2002.

The final construction performed using heavy equipment was the digging of new meander channel between the flow deflector and the upper check dam (this being a flood or overflow channel) and between the upper and lower dams (see Photograph 6). In addition, a shallow pool (less than two feet deep) was dug above each of the dams. The size of these (at low flow) is approximately 20 feet by 30 feet, above the upper dam, and 30 feet by 60 feet above the lower dam. These shallow pools connect with the existing stream channel to provide a considerable increase in open-water area on the site.

Revegetation work (applying and mulching a native grass & rush mixture and planting trees and shrubs) followed the heavy construction during November. Finally, a foot bridge was constructed by Tribal Fisheries staff over the upper check dam in December (see Photograph 7).